Optical fiber is generally produced by melting and drawing a fiber from a quartz glass (glass perform). Specifically, the glass preform is melted at a high temperature of about 2000° C., and glass that has melted from the tip thereof is drawn into a fiber. The drawn glass is then rapidly cooled in a cooling duct or the like, and then the surface of the fiber is coated with a protective resin, thereby completing production.
In this optical fiber production process, when the molten and drawn glass is rapidly cooled, non-bridging oxygen hole centers (NBOHC) are known to form. By slowing the cooling speed of the molten and drawn glass, the recombination of the NBOHCs is promoted. The number of such defects can be reduced. However, in the optical fiber production process, it is difficult to completely eliminate NBOHCs from an optical fiber.
The NBOHCs that remain in the optical fiber combine with hydrogen that evolves from the protective resin on the surface of the optical fiber or from the cable armor and the environment in which the optical fiber cable has been laid, becoming Si—OH. When such hydroxyl groups (—OH) form, optical loss at a wavelength of 1.38 μm increases, compromising the optical fiber propagation characteristics.
One approach for suppressing optical loss at 1.38 μm involves exposing the optical fiber to deuterium-containing gas so as to effect a reaction between NBOHCs in the glass with deuterium (D2) in order to form deuteroxyl groups (—OD) (see JP-A 2002-148450, JP-A 2003-137580).
NBOHCs can readily react with deuterium at room temperature to form deuteroxyl groups. Because the light absorption by these deuteroxyl groups occurs at a wavelength of 1.87 μm, the absorption wavelength band can be shifted outside of the 1.3 μm optical communication wavelength band.
In the foregoing method of reacting NBOHCs with deuterium (D2) to form deuteroxyl groups (—OD), in order to hold down production costs, it is desirable for treatment to be carried out over a short exposure time using a gas containing a low concentration of deuterium.
For example, JP-A 2002-148450 discloses that an exposure time of one week is required to expose optical fiber to deuterium-containing gas. The optical fiber is generally exposed to the deuterium-containing gas in a bobbin-wound state. However, the deuterium-containing gas has difficulty reaching the area close to the core of the bobbin. Since NBOHCs in the optical fiber readily combine with deuterium to form deuteroxyl groups, the reaction for rendering NBOHCs into deuteroxyl groups is strongly dependent on the amount of deuterium molecules that penetrate to the optical fiber.
For such reasons, of the optical fiber that has been wound onto a bobbin, penetration of the deuterium molecules into the optical fiber located near the bobbin core, i.e., in the bottom layers of the optical fiber, takes time, making it difficult for the reaction between the NBOHCs and deuterium to proceed. For example, even when an optical fiber that has been wound onto a bobbin or the like is exposed for 24 hours to a mixed gas containing deuterium gas in a concentration of 1% in accordance with the method disclosed in the foregoing reference, depending on the amount of optical fiber wound onto the bobbin, the NBOHCs are not eliminated in the optical fiber located near the core of the bobbin, i.e., in the bottom layers. Therefore, eliminating NBOHCs throughout the optical fiber has required a high-concentration deuterium-containing gas and a long exposure time.
The exposure of optical fiber to a deuterium-containing gas atmosphere is carried out by placing the optical fiber in a reactor, feeding deuterium-containing gas into the reactor, and then sealing the reactor to maintain the exposure for a given period of time in this state.
Following exposure of the optical fiber for a given period of time to the deuterium-containing gas atmosphere, it has been the practice until now to discharge the deuterium-containing gas without re-using it. However, due to the high cost of deuterium-containing gas, this practice increases the production cost of the optical fiber.